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What temp is most critical at cold side?

Well...

Messing with a chiller, the mixture is Dowcal N 25%, approx freezing temp for the mix is -10degC/14degF. Measuring in the plate exchanger where the temp probe is placed i got -9degC/15,8degF, on outgoing water there's -6,5degC/20,3degF, can i turn down the thermostat (Carel PCO) or can the exchanger freeze over if i do so? The chiller is serving a bunch of disks at a grocery store, the manager want a li'l bit colder in a pair of disks, by this i have to lower the outgoing temp.

By general. Which temperature is most critical? Exch. temp or outgoing temp?

the coldest temperature is the most important! whatever temperature you turn the thermostat to, please remember that the sensors do not know what the freeze point is, so the closer you run to the freeze point, the more you will want to check the fluids glycol concentration.

Talked to my foreman 'bout this. He wanted me to move the probes to the in and outlet pipes instead of the exchanger position. But I didn't do this a thursday afternoon and overtime, you know Murphy and his laws? Also told him 'bout the issue with running too cold. He mentioned that the brine will thicken and loose circulation, but when that's happening the exchanger freeze is a fact. The outlet probes also works as freeze guards. Stupid...

Anyhow... Will check the VKA:s coarse(? correct word?) and clean it if needed. May be clogged with old dirt and stuff...

Checked the dowcal mix with refractometer today. 38%, that'll give us a freezing point at -18degC/-0,4degF. Changed the probe calibration with 2degC/3,6degF so it's getting closer to in and outgoing pipe temp. Also lowered the inlet temp regulation with 1degC/1,8degF, when the chiller unloaded fully I had a outgoing pipe around -10degC/14degF, incoming was -4,5degC/23,9degF.

Leaved the groce store satisfied. But amazingly surprised later same day, my foreman called, the disks was warm... Got there. Yup... Antifreeze alarm..., kicks in by -15degC/5degF. Reseted, fired it up, changed probe calib and inlet to more safe values. Let's se how this will appear